.gitlab-ci.yml: Replace the cobertura tag with coverage_report
.gitlab-ci.yml: Replace the cobertura tag with coverage_report

Gitlab 15 changed artifacts:reports:cobertura for a slightly more
verbose syntax.

https://docs.gitlab.com/ee/ci/yaml/artifacts_reports.html#artifactsreportscobertura-removed

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/711>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg